The present invention relates to an opaque, biaxially oriented polyhydroxycarboxylic acid film comprising at least one layer comprising a polymer based on hydroxycarboxylic acids and a cycloolefin copolymer (COC). The invention further relates to a process for producing the PHC film and the use thereof.
Opaque, biaxially oriented films are known in prior art. These films are characterized by a glossy, white, pearly visual appearance that is desirable for certain applications. In addition, such films have a reduced density allowing the user to obtain an increased yield.
EP 1 068 949 (DE 199 32 384) describes a white, biaxially oriented PET film having at least one layer comprising a cycloolefin copolymer (COC) in a concentration of 2 to 60% by weight, based on the layer. The glass transition temperature of the cycloolefin copolymer (COC) is in the range of 70 to 270° C. It is described that the COC leads to a whitening of the PET film. Simultaneous orientation, i.e. orienting at the same time in both directions (machine direction and transverse direction), to produce the film is not recommended since this simultaneous stretching does not lead to the desired degree of whiteness.
WO 03/033574 describes the production of an opaque film having vacuoles using simultaneous stretching methods. These films are made of polypropylene and comprise special inorganic vacuole-initiating particles. It is described that the particles must have a special shape or a special size distribution in order to initiate vacuoles despite simultaneous stretching. It is indicated that the particles must be rod-like or platelet-like. Alternatively, spherical particles can also be used if they have a minimum size of 3 μm and a narrow size distribution.
EP 1 112 167 describes problems occurring when known technologies are transferred from the sequential stenter process to simultaneous stretching methods. In particular, incompatible additives do not generate vacuoles in polypropylene film during production by means of simultaneous stretching methods, as is the case with sequential orientation. EP 1 112 167 describes solving this problem by using foaming agents in the simultaneous orientation. The foaming agents decompose at the extrusion temperatures and during simultaneous orientation lead to small gas-filled bubbles, similar to the vacuoles generated by incompatible particles. However, the opacity and degree of whiteness of these polypropylene films is very unsatisfactory.
WO 02/088230 describes opaque, biaxially oriented PLA films comprising in at least one layer 0.5 to 30% by weight of COC having a glass transition temperature of 70 to 270° C. This special COC causes the formation of vacuoles during the production of the PLA film by means of sequential biaxial orientation. Further methods for producing the PLA film are not specified. The mechanical properties of the film are in need of improvement.
In the search for further vacuole-initiating additives for PLA films, it became apparent that the mechanisms during vacuole formation in a PLA matrix are different from those in a polypropylene matrix. With PLA films, not just the incompatibility of the particles seems to be important since particles incompatible with the PLA, such as for example CaCO3 or polypropylene, do not lead to vacuole formation or do so only to a completely insufficient extent during biaxial orientation. To date, the COC polymers according to WO 02/088230 are de facto the only known effective vacuole formers in biaxially oriented PLA films.
In principle, the formation of vacuoles is based on the generation of microcracks at the interface between the polymer and the particulate additive during longitudinal stretching. During subsequent transverse stretching, these fine longitudinal cracks tear open to form air-filled, closed hollow spaces. Hence, it seems plausible that the generation of vacuoles during simultaneous orientation is disproportionately more difficult than during sequential orientation. Indeed, it becomes apparent in practice that the particles incompatible in polypropylene that are common, such as CaCO3 or PBT, do not generate vacuoles at all or generate them only with a selective particle shape or particle size (see WO03/033574) during simultaneous orientation. For this process, an alternative technology for generating vacuoles by means of foaming agents was therefore developed.
Similar problems are known about the production of vacuole-containing films made of polyethylene terephthalate. With these films, the generation of vacuoles in a sequential orientation is also technically much more difficult than with polypropylene films. The LISIM process for producing vacuole-containing PET films is therefore explicitly not recommended (see EP 1 068 949).